NAJAF, Iraq, Jan 21 (Reuters) - Anti-American Shi'ite cleric Moqtada al-Sadr returned to Iran on Friday after spending a couple of weeks testing the political waters back home in Iraq after a long, self-imposed exile, aides said.

Sadr, whose speeches rallied millions of poor Iraqi Shi'ites against U.S. forces after the 2003 invasion and whose militia played a major role in the sectarian carnage that gripped Iraq, slipped out of the country without fanfare.

"Yes, definitely Sayyed Moqtada al-Sadr went back to Iran," a source inside his office said, on condition of anonymity.

It was not immediately clear if the young cleric had returned to Iran temporarily or if he intended to stay a while, perhaps to resume religious studies in the Iranian city of Qom.

One senior former member of Sadr's Mehdi Army militia said his return to Iran was a "surprise" while a member of his political bloc said he was expected to come back soon.

Sadr's return to Iraq on Jan. 5 rattled the political establishment, more than three years after he fled the country facing an old arrest warrant brought against him by U.S. administrators.

Sadr's movement has become a powerful political force in Iraq after winning 39 parliamentary seats in last year's election and playing a pivotal role in securing Prime Minister Nuri al-Maliki's reappointment last month.

His clout will make it difficult for Maliki to contemplate an extension of the U.S. military presence beyond the end of the year, when the U.S. forces that ousted Saddam Hussein in 2003 must withdraw under the terms of a security pact.

Sadr, whose militia battled U.S. troops twice in 2004 but was crushed by Maliki in a U.S.-Iraqi offensive in 2008, demanded in his first public speech after his return that the government honour a promise not to allow U.S. troops to stay.

The number of U.S. troops in Iraq fell below 50,000 last August when the U.S. military switched its role to advising and assisting its Iraqi counterparts, rather than leading the fight against a weakened but still lethal insurgency. (Reporting by Khaled Farhan in Najaf and Suadad al-Salhy in Baghdad; Writing by Michael Christie; Editing by Janet Lawrence)

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